The present invention relates to polymerization catalysts or more particularly to non-antimony containing catalysts for the polymerization of polyesters.
It is known in the art to produce polyesters by a two step process wherein the first step involves either direct esterification or transesterification between a glycol and a diacid to produce an oligomer having up to about ten repeating units. In the second step the oligomer is polymerized to a polyester at elevated temperatures, reduced pressures and in the presence of a polymerization catalyst. Common catalysts for this second step are antimony containing, for example antimony trioxide. While antimony containing catalysts are widely used in the art, as evidenced by U.S. Pat. Nos. 5,434,239 and 5,242,645, they have serious drawbacks such as producing films having black specks, large area polarization defects and pimple rejects. Furthermore, antimony compounds frequently contain impurities which impart an undesired color to the polyester product. They also tend to precipitate or generate an elemental antimony precipitate in the reaction stream causing fouling of reaction vessels and extruding equipment. It would be desirable to be able to produce polyesters using catalysts free of antimony compounds and which do not have all of these shortcomings.
It has now been found that a polymerization catalyst which is a clear, chlorine and/or bromine containing solution of a metal glycoxide and a metal glycolate having a pH in the range of from 0 to about 1 is a much more effective catalyst for the condensation of polyester precursors into polyesters. Polyesters produced with this catalyst have improved melt elasticity and higher melt viscosity. The former property is desirable for the preparation of large and complex shaped polyester containers, and the latter property is desirable for the preparation of tire cords and industrial grade fibers.